As is well known, an image forming apparatus such as a printer or a copy machine is provided with a paper feed device for feeding sheets of paper one by one. The sheet of paper fed by the paper feed device is conveyed to a paper forming unit, in which image is formed. Between the paper feed device and the image forming unit, a pair of registration rollers is arranged for registering leading edge of the sheet and for skew correction. The conveyed sheet is once stopped at the pair of registration rollers for skew correction, and conveyed again to the image forming unit with timing adjusted for alignment with the image to be formed.
In a high-speed apparatus capable of high-speed printing, there is a demand to increase the number of copies per unit time (CPM: Copy Per Minute) as much as possible while not very much increasing the speed of image formation in copy-printing (process speed) (with the process speed kept as low as possible). In order to meet such a demand, it is desirable to make as narrow as possible the space or interval between a preceding sheet and a following, succeeding sheet, when the sheets are fed successively.
Patent Literature 1, which will be described later, proposes a method of controlling paper feeding and conveyance enabling feeding and conveyance of paper without reducing productivity. According to the method of controlling paper feeding and conveyance of Patent Literature 1, when the leading edge of preceding sheet is stopped at the registration roller pair, the trailing edge of the preceding sheet is detected by a sensor provided along a conveying path, and a trigger that determines the timing to start feeding of the next sheet is changed in accordance with the result of detection. Specifically, that the trailing edge of preceding sheet has passed through the sensor, or that re-conveyance of the preceding sheet after temporarily stop at the registration roller pair has started, is used as the trigger for determining the timing to start feeding of the next sheet, depending on the result of detection.
If the passage of trailing edge of preceding paper through the sensor is used as the trigger and the trailing edge of preceding sheet is stopped held on the paper conveying device while the leading edge of the preceding sheet is stopped at the registration roller pair, it follows that the time of passage through the sensor serving as a trigger to start feeding of the next sheet will be after re-conveyance of the preceding sheet by the registration roller pair. In that case, the interval between the sheets becomes too wide, leading to lower productivity.
Therefore, according to Patent Literature 1, if the sensor detects that the trailing edge of preceding sheet is stopped held on paper conveying device, the trigger to start feeding of the next sheet is changed to the start of re-conveyance of the preceding paper after temporary stop at the registration roller pair. In this manner, the interval between sheets of paper is made narrower, to prevent the paper interval from becoming too long.